The present invention relates generally to the field of cancellers for use in sidelobe canceller systems that decorrelate signals by removing correlated components. More specifically, the invention relates to digital open-loop cancellers.
In radar and communication systems, undesired interfering signals can be received via antenna sidelobes. These signals, if powerful enough, can significantly degrade performance. The effect of these interfering signals can be reduced or negated by employing the technique known as coherent sidelobe cancellation (CSLC). A CSLC system utilizes one or more auxiliary antennas and receiving systems, hereafter referred to as auxiliaries. By appropriately weighting and summing the signals received from the auxiliaries and subtracting the sum from the signals received via the radar or communication antenna, hereafter referred to as the main antenna, the interfering signals can in general be cancelled. Typically, these prior art CSLC systems use complex number weighting values in order to achieve sidelobe cancellation. The earliest form of a CSLC system was implemented as an analog closed-loop canceller, as described in U.S. Pat. No. 3,202,990 to Paul W. Howells. This canceller has several disadvantages. The loop gain and effective bandwidth are proportional to auxiliary signal level, the loop gain must be limited to avoid oscillation or instability, and imperfect multipliers permit interference feed through from the auxiliary to radar channels. Several improvements and modifications of the original Howells CSLC have been described, e.g. U.S. Pat. Nos. 3,916,408 to Evans, et al., 3,938,153 to Lewis, et al., and 4,044,359 to Applebaum, et al. The Howells canceller loop has been implemented in digital form as described in the patent to Lewis referred to above but still has the disadvantage that it is a closed loop dependent on auxiliary signal power and loop gain.
A digital open-loop CSLC which does not have the disadvantage of the closed-loop analog system is described in U.S. Pat. No. 4,086,592 to Lewis, et al. This canceller is independent of loop gain and auxiliary signal power and uses digital circuits which do not have drift problems as due auxiliary circuits. Cancellation is performed by first converting the input signals in the main and auxiliary channels to baseband components, digitizing these components, and then treating the digitized baseband components as complex numbers. This canceller has the disadvantage that, because of equipment inaccuracies, the baseband components from each channel do not necessarily represent the true in-phase and quadrature-phase components of the input signals. Thus, the complex representation of the baseband signals results in suboptimal performance.